Field of the Art
The present disclosure relates to an ester-crosslinkable composite hollow fiber membrane prepared using a 4,4′-(hexafluoroisopropylidone)dipthalic anhydride (6FDA)-based polyimide and a polyamide-imide or polyetherimide polymer. More specifically, provided is a composite hollow fiber membrane that demonstrates high separation performance for hydrocarbon contaminants making it useful for natural gas separation.
Description of the Related Art
Natural gas processing occurs prior to sending natural gas to the distribution pipeline for sale. Natural gas processing involves removing impurities such as carbon dioxide (CO2), water (H2O), and hydrogen sulfide (H2S) from natural gas. Acid gas removal refers to removing CO2 and H2S from natural gas, while dehydration refers to removing H2O from natural gas.
Specifications for sales gas require CO2 removal. For example, U.S. pipeline specifications require that sales gas contain ≦2 mol % CO2. CO2 removal processes include solvent (e.g., amine) absorption, cryogenic distillation, adsorption processes, and membrane separation.
Similarly, since both associated natural gas and non-associated natural gas contain water, natural gas must be subjected to H2O removal. The water content of associated and non-associated natural gas can range from below saturation to 100% saturation. Thus, natural gas always requires dehydration. Suitable dehydration processes include absorption (e.g., by glycol or molecular sieves) and membrane separation.
Since H2S can be extremely harmful to human health and corrosive, natural gas is also generally subjected to H2S removal. Amine (e.g., monoethanolamine or diethanolamine) absorption typically removes H2S.
These impurity removal processes typically occur in series. In most cases, dehydration follows CO2 removal because CO2 removal often increases water content. For example, solvent absorption (e.g., amine absorption) is water based and, consequently, saturates the natural gas. As a result, this water saturated gas then requires treatment to remove water. This water removal step, like any additional step in a series of steps, is undesirable because it increases capital costs and operating costs for natural gas processing.
As discussed above, membrane separation is a separation process useful for removing impurities from natural gas. For natural gas applications, membranes having both high CO2/CH4 selectivity and high CO2 permeability are desired. High selectivity for CO2 over CH4 minimizes loss of methane, a valuable component of natural gas, to the permeate stream. High CO2 permeability decreases the required membrane area for a particular separation. However, while membrane separation may be useful and desirable for natural gas applications, it is difficult to produce membranes having both high CO2/CH4 selectivity and high CO2 permeability.
Most commercially available polymer membranes have high CO2 permeability and low to moderate CO2/CH4 selectivity or low to moderate CO2 permeability and high CO2/CH4 selectivity. While some high performance specialty polymer membranes having both high CO2/CH4 selectivity and CO2 permeability have been developed, these specialty membranes are not commercially available and have a high cost.
Utilizing a composite polymer membrane having a thin, selective outer layer of high performance specialty polymer on a porous substructure layer made of an inexpensive polymer reduces membrane cost. While composite polymer membranes are known in the art, they are difficult to produce due to the difficulty in finding compatible polymers for the selective outer layer and the porous substructure layer.
Dual-layer hollow spinning technology is commonly used to produce defect-free composite hollow fibers through the so-called dry-jet/wet-quench process by simultaneous extrusion of two polymer solutions without adding a post-spinning coating step (see L. Y. Jiang, T. S. Chung, D. F. Li, C. Cao, A. Kulprathipanja, Fabrication of Matrimid/polyethersulfone dual-layer hollow fiber membranes for gas separation, J. Membrane Sci., 240 (2004) 91-103; I. Pinnau, J. Wind, K. V. Peinemann, Ultrathin Multicomponent Poly(Ether Sulfone) Membranes for Gas Separation Made by Dry Wet Phase Inversion, Ind. Eng. Chem. Res., 29 (1990) 2028-2032; S. C. Pesek, W. J. Koros, Aqueous Quenched Asymmetric Polysulfone Membranes Prepared by Dry Wet Phase-Separation, J. Membrane Sci., 81 (1993) 71-83; and S. Husain, W. J. Koros, Mixed matrix hollow fiber membranes made with modified HSSZ-13 zeolite in polyetherimide polymer matrix for gas separation, J. Membrane Sci., 288 (2007) 195-207). This efficient process provides economical alternative defect-free hollow fibers that eliminate instability in “caulked” hollow fibers under aggressive feed conditions (D. W. Wallace, Crosslinked Hollow Fiber Membranes for Natural Gas Purification and Their Manufacture from Novel Polymers, Ph.D. Dissertation, in: Chemical Engineering, The University of Texas at Austin, Austin, Tex., 2004). Composite hollow fiber membranes, therefore, combine the advantages of lower cost polymers as the supporting non-selective core layer and high performance polymer as the selective sheath layer (C. C. Pereira, R. Nobrega, K. V. Peinemann, C. P. Borges, Hollow fiber membranes obtained by simultaneous spinning of two polymer solutions: a morphological study, J. Membrane Sci., 226 (2003) 35-50; and H. Strathmann, Membrane separation processes: Current relevance and future opportunities, AIChEJ, 47 (2001) 1077-1087). Ideally, the core layer provides the mechanical strength to withstand high transmembrane pressure difference and has negligible transport resistance for gas separations, while the sheath layer serves as the selective layer, which allows a high separation productivity and efficiency (D. F. Li, T. S. Chung, R. Wang, Y. Liu, Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation, J. Membrane Sci., 198 (2002) 211-223). The significantly reduced cost of membrane formation with high separation performance makes dual-layer hollow fiber spinning especially attractive for large scale gas separations that require large membrane areas with high feed pressures (J. Liu, Development of Next Generation Mixed Matrix Hollow Fiber Membranes for Butane Isomer Separation, PhD Dissertation, in: School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Ga., 2010).
Development of composite hollow fiber membranes dates back to Henne et al, who disclosed a dual-layer composite hollow fiber membrane for hemodialysis (D. F. Li, T. S. Chung, W. Rong, Morphological aspects and structure control of dual-layer asymmetric hollow fiber membranes formed by a simultaneous co-extrusion approach, J. Membrane Sci., 243 (2004) 155-175; and W. Henne, G. Dunweg, W. Schmitz, R. Pohle, F. Lawitzki, Method of producing dialyzing membrane, in, U.S. Pat. No. 4,164,437, 1979). The first set of composite hollow fiber membranes used for gas separations were disclosed by Du Pont in 1992 (D. F. Li, T. S. Chung, R. Wang, Y. Liu, Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation, J. Membrane Sci., 198 (2002) 211-223). As for natural gas separations, Jiang et al fabricated Matrimid/polyethersulfone dual-layer hollow fibers with a CO2/CH4 selectivity around 40 tested with 120˜270 psi 40/60 CO2/CH4 at 22° C. However, the maximum achieved CO2 permeance was only up to 11 GPU (L. Y. Jiang, T. S. Chung, D. F. Li, C. Cao, A. Kulprathipanja, Fabrication of Matrimid/polyethersulfone dual-layer hollow fiber membranes for gas separation, J. Membrane Sci., 240 (2004) 91-103; Li et al distributed PES-zeolite into dual-layer polyethersulfone (PES)/BTDA-TDI/MDI co-polyimide (P84) hollow fiber membranes to enhance CO2/CH4 selectivity (Y. Li, T. S. Chung, Z. Huang, S. Kulprathipanja, Dual-layer polyethersulfone (PES)/BTDA-TDI/MDI co-polyimide (P84) hollow fiber membranes with a submicron PES-zeolite beta mixed matrix dense-selective layer for gas separation, J. Membrane Sci., 277 (2006) 28-37); however, further heat treatment and additional coating resulted in a CO2 permeance lower than 0.164 GPU and a CO2/CH4 selectivity below 33.4 tested with 190 psig of 50/50 CO2/CH4 at 24° C. Besides the lower separation productivity, delamination of sheath/core layers can significantly undermine the mechanical strength under high feed pressures, which has been discussed in fluoropolyimide/polyethersulfone (PES) dual-layer hollow fibers (D. F. Li, T. S. Chung, R. Wang, Y. Liu, Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation, J. Membrane Sci., 198 (2002) 211-223). Lower CO2 permeance (membrane separation productivity) indicates that a high-performance polymer material as the selective sheath layer, as well as a robust core layer polymer, is needed to achieve high permeate flux and separation efficacy under aggressive feed conditions.
Liu et al. applied chemical crosslinking modification on polyimide/poly (ether sulfone) dual-layer hollow fibers but the chemical crosslinked hollow fibers tend to plasticize under a CO2 feed pressure ˜50 psi (Y. Liu, T. S. Chung, R. Wang, D. F. Li, M. L. Chng, Chemical cross-linking modification of polyimide/poly(ether sulfone) dual-layer hollow-fiber membranes for gas separation, Ind. Eng. Chem. Res., 42 (2003) 1190-1195). Researchers demonstrated that 6FDA-based crosslinkable polyimide hollow fibers showed a CO2 permeance over 50 GPU and a CO2/CH4 selectivity above 40 tested with 200 psi of 50/50 CO2/CH4 at 35° C. (I. C. Omole, R. T. Adams, S. J. Miller, W. J. Koros, Effects of CO2 on a High Performance Hollow-Fiber Membrane for Natural Gas Purification, Ind. Eng. Chem. Res., 49 (2010) 4887-4896; I. C. Omole, S. J. Miller, W. J. Koros, Increased molecular weight of a cross-linkable polyimide for spinning plasticization resistant hollow fiber membranes, Macromolecules, 41 (2008) 6367-6375; I. C. Omole, Crosslinked Polyimide Hollow Fiber Membranes for Aggressive Natural Gas Feed Streams, Ph.D. Dissertation, in: Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Ga., 2008; I. C. Omole, D. A. Bhandari, S. J. Miller, W. J. Koros, Toluene impurity effects on CO2 separation using a hollow fiber membrane for natural gas, J. Membrane Sci., 369 (2011) 490-498). Despite the good separation performance, the high cost of 6FDA-based crosslinkable polyimide increases cost. Dual-layer hollow fiber spinning technique can be utilized to reduce the amount of expensive polyimide required, while maintaining high separation performance. Despite the attractive aspects of such advanced membranes, it is challenging to integrate a low-cost polymer core layer with the sheath layer to develop crosslinkable composite hollow fibers. A key challenge to overcome in such a membrane is the potential delamination of sheath/core layers and the collapse of the core layer polymer during aggressive heat treatment to crosslink the high performance sheath layer (C. C. Pereira, R. Nobrega, K. V. Peinemann, C. P. Borges, Hollow fiber membranes obtained by simultaneous spinning of two polymer solutions: a morphological study, J. Membrane Sci., 226 (2003) 35-50; and D. F. Li, T. S. Chung, R. Wang, Y. Liu, Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation, J. Membrane Sci., 198 (2002) 211-223).
The so-called sorption-diffusion model applies to polymeric gas separation membranes. In this model, the gas permeants first sorb in the upstream of a membrane and then diffuse through the membrane under a partial pressure/fugacity difference. The differences in the amount of gas sorbed in the membrane and the permeant diffusion rate through the membrane cause the gas mixture to be separated. In this case, the permeability of a polymer membrane, P, can be described by the product of the diffusion coefficient, D, and sorption coefficients, S, as shown in Equation 1 (W. J. Koros, G. K. Fleming, Membrane-Based Gas Separation, J. Membrane Sci., 83 (1993) 1-80).P=D·S  (1)
The upstream sorption coefficient, S, in glassy polymer membranes can be described well by the so-called dual-mode model, shown in Equation 2 (W. J. Koros, G. K. Fleming, Membrane-Based Gas Separation, J. Membrane Sci., 83 (1993) 1-80).
                                          C            A                                p            A                          =                              S            A                    =                                    k              Di                        +                                                            C                  Hi                  ′                                ⁢                                  b                  i                                                            1                +                                                      b                    A                                    ⁢                                      p                    A                                                  +                                                      b                    B                                    ⁢                                      p                    B                                                                                                          (        2        )            
In Equation 2, kDi is the Henry's law constant, C′Hi is the Langmuir capacity constant, bi is the Langmuir affinity constant, and pi is the local effective partial pressure of component i, which represents the local chemical potential for component i.
To characterize the separation performance of a hollow fiber membrane, two key factors, termed as permeance and selectivity, can be considered. The permeance, Pi/l, represents the separation productivity of a hollow fiber membrane and is defined as the flux of penetrant i normalized by the partial pressure or fugacity difference across the membrane, as shown in Equation 3.
                                          P            i                    l                =                              n            i                                Δ            ⁢                                                  ⁢                          p              i                                                          (        3        )            
In Equation 3, Pi represents the permeability of penetrant i; l describes the effective membrane thickness; ni represents the flux of penetrant i through the membrane; Δp refers the partial pressure or fugacity difference of each penetrant across the membrane. The common unit of permeance is the GPU, which is defined as Equation 4.
                              G          ⁢                                          ⁢          P          ⁢                                          ⁢          U                =                              10                          -              6                                ⁢                      (                                          cc                ⁡                                  (                                      S                    ⁢                                                                                  ⁢                    T                    ⁢                                                                                  ⁢                    P                                    )                                                                              cm                  2                                ·                s                ·                cmHg                                      )                                              (        4        )            
The selectivity, αij, measures the membrane separation efficacy for a gas pair under conditions where the upstream pressure is much greater than the downstream, as it is in this study. It is defined by the ratio of the fast gas (i) permeance to the slow gas (j) permeance, as shown in Equation 5.
                              α          ij                =                                            P              i                        /            l                                              P              j                        /            l                                              (        5        )            
As a key challenge for conventional polymeric membranes, the plasticization of polymeric membranes is often observed when an elevated feeding CO2 partial pressure increases the permeance but reduces selectivity significantly. To develop a robust membrane with solid separation performance, the CO2 induced plasticization must be suppressed to achieve a high permeance without loss of selectivity. Past studies have demonstrated that a highly effective approach, ester-crosslinking, can improve the CO2/CH4 selectivity and CO2 plasticization resistance of a polymer by reducing the degree of swelling and segmental chain mobility in the polymer.
While dual-layer hollow fiber spinning reduces the usage of expensive crosslinkable sheath polymer by up to 90%, crosslinking of composite hollow fibers can cause a delamination of sheath/core layers. Moreover, aggressive heat treatment during crosslinking tends to cause a collapse of core layer polymer and to reduce the permeance significantly.
Achieving a low-cost, defect-free crosslinked composite hollow fiber without delamination, while also exhibiting improved productivity and plasticization resistance, would be of great value to the industry. Such a hollow fiber membrane would find great use economically and effectively in purifying natural gas.